The present invention relates to a power supply noise sensor for sensing impulse noise or glitch which causes, when applied to a power supply line of a digital system, an erroneous operation in the digital system.
Digital systems including digital circuits have been today put to use in various fields. It generally considered that a digital circuit is more resistive against noise than an analog circuit. However, when the noise exceeds a limit value of error acceptable (noise immunity) for the digital circuit, a fatal error possibly occurs to considerably damage the digital circuit in some cases. On the other hand, most erroneous operations of digital systems are caused by noise entering from a signal line and a power supply line. Reference is to be made to xe2x80x9cNoise of Control Devices and Countermeasure against Noisexe2x80x9d written in IEEJ Technical Report II, No. 123 published in 1979.
Particularly, when noise enters a digital circuit from a power supply side, it cannot be forecasted that an output is at a high (H) level or a low (L) level. This makes it more difficult to appropriately cope with the noise. To implement a system of fail safe type for failures in a digital circuit, a redundant system including a backup system is generally incorporated in the fail-safe system. However, the power supply noise similarly causes an erroneous operation also in the backup system, and hence the backup system does not satisfactorily function to implement a fail-safe system depending on cases.
According to a method to implement a fail-safe system against noise superimposed on the power supply system, when a power supply noise is detected, the digital system is controlled, according to the detected signal, to be set to a safe mode, namely, a state in which an erroneous operation does not take place. A device to detect the noise superimposed onto the power supply system is, for example, xe2x80x9ca power supply monitor circuitxe2x80x9d described in Japanese Patent Application Laid-Open No. HEI 9-054620 or xe2x80x9ca power supply noise detector circuitxe2x80x9d described in Japanese Patent No. 2953761. According to the Japanese Patent Application Laid-Open No. HEI 9-054620, a delay circuit and a through-rate detector circuit is combined with each other to prevent an erroneous operation of the power supply monitor circuit such that a reset signal is produced when power is lowered to a value equal to or less than a predetermined through rate or when a low-power state continues for a period of time equal to or more than a predetermined period of time. On the other hand, according to Japanese Patent No. 2953761, an asynchronous input sensitive to noise, namely, a clock input terminal of a flipxe2x80x94flip circuit is connected to a power supply such that the flip-flop circuit is inverted or activated by noise superimposed onto the power supply. Moreover, Japanese Patent No. 2703890 describes xe2x80x9ca semiconductor integrated circuitxe2x80x9d associated with improvement of a reset unit which implements a power-on operation when a power supply voltage is considerably lowered, for example, by an instantaneous interruption of power. This circuit also includes a power-on detector circuit which generates a one-shot pulse in response to power-on operation.
However, there remain problems that the xe2x80x9cpower supply monitor circuitxe2x80x9d detects a variation in a power supply signal with a relatively low frequency, and on the other hand, the flip-flop circuit of the xe2x80x9cpower supply noise detector circuitxe2x80x9d may conduct an erroneous operation when power supply noise is received. Additionally, the xe2x80x9csemiconductor integrated circuitxe2x80x9d operates in a state in which the output signal from the power supply is at a low level; however, any countermeasure is described against impulse noise in a positive direction. Moreover, either one of the above techniques to detect power supply noise is attended with a problem that the circuit configuration becomes complex. Impulse noise is in general in a damped oscillation when the noise reaches the LSI, and hence the noise varies in both directions, namely, in the negative and positive directions. Therefore, the technique to cope with the impulse noise in only one direction, namely, the negative or positive direction cannot be satisfactory in practical applications.
It is therefore an object of the present invention to provide a power supply noise sensor constructed in a simple configuration capable of sensing or detecting a variation in the power supply output signal with a high frequency in which impulse noise in the position direction and impulse noise in the negative direction superimposed onto a power supply system of the power supply noise sensor can be sensed with high reliability.
In accordance with the present invention, there is provided a power supply noise sensor, comprising a noise sensing section for sensing impulse noise applied to a power supply line; and an output holding section for holding a sense output from said noise sensing section, said noise sensing section including a time constant circuit for gradually varying a power supply voltage including impulse noise, wherein when a peak value of the impulse noise reduces after the peak value exceeds a predetermined value, a direction of a current flowing through said time constant circuit is inverted to hold a noise output from said output holding section also after the impulse noise disappears. Therefore, the impulse noise superimposed onto the power supply line can be appropriately sensed, and hence each circuit can be easily protected.
Moreover, in said power supply noise sensor, said noise sensing section comprises a first resistor, a second resistor, and a third resistor connected in series between a power supply terminal and ground; a capacitor forming said time constant circuit together with said first and second resistors, said capacitor being connected in parallel to said second resistor; and a first transistor including a base, a collector, and an emitter, the base being connected to a connecting point between said second and third resistors, the collector being connected via a fourth resistor to said power supply terminal, said emitter being connected to ground. In consequence, the noise sensor can be implemented in a simple circuit configuration.
Additionally, in said power supply noise sensor, said output holding section comprises a fifth resistor and a sixth resistor connected in series between ground and a connecting point between said fourth resistor and the collector of said first transistor; and a second transistor including a base, a collector, and an emitter, the base being connected to a connecting point between said fifth and sixth resistors, the collector being connected to a connecting point between said first and second resistors, said emitter being connected to ground. As a result, the noise sensing and holding operation can be achieved using a simple circuit configuration.
Furthermore, in said power supply noise sensor, a base current of said first transistor is larger after the sensing of the impulse noise than before the sensing thereof. Therefore, the noise can be sensed with higher reliability.
Moreover, said power supply noise sensor further comprises a reset circuit for turning said first transistor on at power on or after the sensing of the impulse noise, said reset circuit being a first switch circuit for lowing a base potential of said second transistor to a potential of ground. Consequently, after the detection of the noise, the state of the noise sensor can be restored to the noise sense ready state in any situation.
Additionally, said power supply noise sensor further comprises a reset circuit for turning said first transistor on at power on or after the sensing of the impulse noise, said reset circuit being a second switch circuit for connecting the collector of said second transistor to a power supply terminal. Therefore, the state of the noise sensor can be recovered after the noise detection to the noise sense ready state in any situation.
Further more, said power supply noise sensor further comprises a reset circuit for turning said first transistor on at power on or after the sensing of the impulse noise, said reset circuit being a third switch circuit for lowing a collector potential of said first transistor to a potential of ground. In consequence, after the noise is detected, the noise sensor can be restored to the noise sense ready state in any situation.
In accordance with the present invention, there is provided a power supply noise sensor, comprising a parallel connection including a plurality of sets each of which includes said noise sensing section and said output holding section, said sets each including said noise sensing section and said output holding section having mutually different time constants. In the configuration, there can be implemented a noise sensing operation with different sensitivity for impulse noise, that is, noise can be sensed according to different magnitude thereof.